1. Field of the Invention
The present invention relates to an image forming apparatus such as a color copier or color printer which uses an electrophotographic process.
2. Description of the Related Art
An electrophotographic process using a toner is often used for an image forming apparatus such as a copier, laser printer, or facsimile machine. The electrophotographic process includes a transfer step of transferring toner images, i.e. developer images, carried on a photosensitive drum which acts as an image bearing member to a surface of a transfer material conveyed by a transfer material conveying belt or intermediate transfer belt. A transfer belt such as a transfer material conveying belt or intermediate transfer belt are stretched around multiple tension rollers and configured to rotate and move when a driving roller rotates. The transfer belt could move to one side along a belt width direction orthogonal to a rotational direction during the rotating movement. For restricting a movement in the belt width direction crossing the rotational direction of the belt, in Japanese Patent Application Laid-Open No. H11-223971, ribs are installed as guide portions along opposite ends of an inner circumferential surface of the transfer belt. The ribs are abutted against flanges provided on a driving roller, and the movement in the belt width direction crossing the rotational direction of the belt is restricted by the ribs.
In Japanese Patent Application Laid-Open No. 2003-215943, a rib is installed along one end of an inner circumferential surface of a transfer belt and rib guides (grooves) are installed at one end of multiple rollers (a driving roller and driven roller) around which an intermediate transfer belt is stretched. The grooves guide the rib by putting the rib therein so that the ends of the belt will not bend inward due to belt tension. The rib guide placed on the driven roller restricts the belt-move in the belt width direction. The rib guide placed on the driving roller is movable by a predetermined amount in the belt width direction, and thus does not block the movement of the rib in the belt width direction.
Normally, to stably convey the transfer belt, a rubber layer with a high coefficient of friction μ is provided on a surface of the driving roller which drives the transfer belt. In Japanese Patent Application Laid-Open No. H11-223971, a moving force on the driving roller in the belt width direction is increased. When restriction forces of rib guides on the ribs increase, loads on the belt and ribs increase. Consequently, ribs of the transfer belt become less durable, resulting in breakage in some cases.
The belt-move is restricted as the belt bumps against the flanges which serve as restriction members. Consequently, when the moving force increases, the ribs may run on the flanges. If the ribs run on the flanges, the transfer belt floats up minutely from a surface of the driving roller, causing changes to driving diameter of the belt and thereby resulting in changes in rotational speed of the transfer belt. This destabilizes position detection of the transfer belt, making it difficult to synchronize image write positions and thereby resulting in color misregistration.
In Japanese Patent Application Laid-Open No. 2003-215943, the use of an intermediate transfer belt with a short circumferential length results in an increased travel amount D of the belt in the belt width direction on the driving roller, increasing a D/B ratio, where B is a center distance between the driving roller and driven roller. This in turn increases an amount of diagonal deformation of the belt itself, causing the belt to undulate and thereby affecting images. To solve this problem, it is conceivable to use high-rigidity material such as polyimide for the intermediate transfer belt or perform feedback control of alignment of the driving roller by using an actuator to prevent the belt from moving in the belt width direction, but these methods will result in cost increases.